1. Technical Field
The present invention relates to a flat panel display apparatus that employs light emitting elements whose luminance mainly changes according to a current, and controls the quantity of electric charge which intermittently flows into a light emitting section to adjust the illumination luminance, and more particularly to a flat panel display apparatus that is capable of suppressing a luminance variation which is caused by a difference in an electron emission start voltage which is a threshold value at which electrons are emitted from a cathode that is an electron source to start the illumination.
2. Technical Background
There exists a current driven display element having an illumination intensity determined according to the quantity of electric charge that is inputted to an illumination layer from an electron emission source within a given period of time, that is, according to a current. As examples of the current driven display element, there are a field emission display (hereinafter referred to as “FED”), and an organic electro luminescence display (hereinafter referred to as “organic EL”).
The FED irradiate a phosphor screen with an electron beam from a large number of cold cathode electron sources that are formed for each of plurality of pixels through a vacuum, to thereby obtain illumination.
Also, there are several types of FEDs which are classified by the electron sources to be applied, such as a Spindt type using a fine conical electron source, a type using electron sources that are called “surface conduction type”, a type using MIM electron sources with an ultrathin film of an oxide film, and a CNT-FED using a carbon nano tube (hereinafter referred to as “CNT”). Even in the case using any electron sources, the illumination intensity is determined according to a voltage of the phosphor screen that is an illumination layer, and the quantity of irradiation of electron beams onto the phosphor screen within a given period of time, that is, a current.
Since a high voltage of several kV or higher is employed as the voltage of the phosphor screen from the viewpoint of the characteristic of a phosphor, it is general to apply a DC voltage, and the luminance of the FED changes according to the quantity of incident electron beams that is a phosphor screen current thereof.
Under the circumstances, the quantity of incident electron beams is determined by changing the electron emission quantity from the electron sources, and for example, in the Spindt type or the CNT-FED, the electron emission quantity from the electron sources is controlled by applying an appropriate voltage to a cathode and a control electrode.
Also, the MIN type or the surface conduction type is not configured by the cathode and the control electrode, and both of those types extract a part of current that flows by applying a voltage between two electrodes to vacuum as electron emission.
On the other hand, the organic EL injects electrons from the cathode and electron holes from the anode into an illumination layer that is formed in each of pixels, to thereby obtain illumination. An energy that is developed by recombination of the electrons and the electron holes which have been injected into the illumination layer that is an organic thin film together causes an exciting state within the illumination layer, and the exciting state is relieved to perform the illumination. Therefore, the illumination intensity of the organic EL is roughly determined according to the number of electrons and electron holes which are injected into the illumination layer within a unit time.
That is, the illumination intensity is determined according to a current that flows in the illumination layer from the anode toward the cathode, and it is general that the illumination intensity is controlled according to the voltage that is applied to the anode and the cathode (hereinafter, the electron emission from the cathode in the FED and the electron injection from the anode in the organic EL are called “electron emission”.
As described above, both of the FED and the organic EL are driven by the voltage by applying a given electrode voltage although the illumination intensity of those elements is determined according to the current. In this case, a difference of the electrode voltage to electron emission characteristics in each of the plurality of pixels is affected, and there is the possibility that a difference occurs in the luminance between the respective pixels even in the case where a given electrode voltage is applied.
In order to prevent the above drawback, it is studied to directly control the current that flows in the elements, and a conventional art that applies the direct control of the current to the organic EL is disclosed in Japanese Patent Laid-Open No. H11-231834.
In Japanese Patent Laid-Open No. H11-231834, the illumination intensity of the organic EL is controlled by driving the organic EL by means of a constant current source that is connected to the cathode. Further, a floating capacitance is charged by another constant current source of the large capacitance or a constant voltage source at the time of transiting from non-selection to selection in the respective cathodes. As a result, a period of time required to charge the floating capacitance is shortened, and the rising characteristic of illumination at the time of selecting the cathode is so improved as to enhance the response.
Also, a display element such as the FED or the organic EL has a matrix structure, and uses a linear sequential display method in which any one of two kinds of electrodes that constitute a matrix is sequentially selected.
The above driving method includes the combination of two states consisting of a selection period that is a short period of period and a non-selection period that is a relatively long period of time in the respective pixels. Because one selection period is short in the period of time, it is difficult for an observer to recognize a change in the luminance in the selection period. Therefore, even in the case where illumination is conducted with a constant luminance during the selection period, or even in the case where illumination is intensely conducted in a short period of time during the selection period, they are recognized as the same luminance if the luminance integration within one selection period is identical with each other.
Japanese Patent Laid-Open No. 2000-133116 discloses a conventional art that applies, to the FED, a method in which the total quantity of charge that flows into the cathode from a cathode power source is controlled by using the above phenomenon within one selection period to control the integrated illumination intensity within one selection period. Also, Japanese Patent Laid-Open No. 2002-23688 discloses a conventional art that also applies the above method to the organic EL. Those conventional arts use a method of emitting electric charges that have been accumulated once in the floating capacitance or an external capacitative element from the cathode in a pulsed fashion.
The display element such as the FED or the organic EL is naturally large in areas where electrodes are disposed opposite to each other because of the provision of a matrix structure, and has a floating capacitance in each of the electrodes. In addition, the display element is capable of correcting the capacitance of the electrodes by the aid of an external capacitance. A reduction in the variation of the total capacitance is easily conducted as compared with a reduction in the variation of the voltage-current characteristics of the electron emission element, thereby making it possible to reduce a luminance variation of the respective pixels. In addition, because the electric charges that are accumulated in the known capacitative element are determined according to a charging voltage that is applied to the capacitative element, it is possible to use a constant voltage source that is simple in the structure for driving.
FIG. 3 shows an inter-electrode voltage-electron emission characteristic, that is, a so-called voltage-current characteristic of the FED that is an object of the present invention, and FIG. 11 shows an example of the inter-electrode voltage-element current characteristic of the organic EL.
In any of the elements, as shown in FIG. 3, an electron emission start voltage is developed between a control electrode and a cathode in the FED, and as shown in FIG. 11, a threshold value indicative of an illumination start voltage exists in the inter-electrode voltage between the anode and the cathode in the organic EL. Each of those elements has such a characteristic that no current flows in the element when the voltage is equal to or lower than the threshold value, and a current starts to rapidly flow in the element when the voltage exceeds the threshold value to perform illumination (hereinafter referred to as “electron emission start voltage”including the illumination start voltage since the illumination starts due to the electron emission from the cathode even in the organic EL element shown in FIG. 11).
As described above, in order to cause the electron emission from the cathode, it is necessary to supply the sum of an electric charge Qc required until the inter-electrode voltage reaches the electron emission start voltage and an electric charge Qe required to obtain the illumination with a given luminance. Because the electric charge Qc is greatly affected by the floating capacitances of cathode lines and anode lines, a variation in the thickness of an insulating film which determines the floating capacitance in the electric charge Qc affects the required quantity of electric charge Qc.
Under the above circumstances, Japanese Patent Laid-Open No. 2002-55652 discloses a conventional art that combines an electron emission start voltage setting corresponding to the variation in the thickness of the insulating film with the electric charge injection for emission to suppress the variation in the luminance of the respective pixels.
In the above conventional art, in a first period of the pixel selection time, the cathode is applied with voltage V1 so that the inter-electrode voltage is slightly lower than the electron emission start voltage even if the electron emission voltage is applied to the control electrode while the electron emission suppression voltage is applied to the control electrode, and electrically charged.
Then, after a voltage for charging the electric charge Qe to be further emitted is applied to the cathode electrode in addition to the voltage V1, the electron emission voltage is applied to the control electrode. As a result, the electron emission that is improved in the uniformity can be performed by only the voltage source.